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[Anual Report 96 :
Table of Contents]

Effects of Human Disturbance on
Genetic Diversity in Tropical Forests

Harvesting timber from forests contributes significantly to the GNP of many countries. However, many more people depend on the forest in less lucrative but more intimate ways, deriving part or most of their livelihoods from harvesting non-timber forest products, or using the forest for livestock grazing and other activities. This is especially true in the tropics.

Tropical forests are estimated to contain more than 50 per cent of all terrestrial biodiversity. The way these are used therefore has very significant implications for global biodiversity. As the forests are so critical for the survival and welfare of so many people in the tropics, conservation of biodiversity must be compatible with continued economic benefits being derived from these forests. It is necessary to achieve a balance between these potentially conflicting demands to ensure that all facets of sustainability ­ ecological, economic and social ­ are met. Such a balance requires an understanding of the impacts of harvesting on the genetic resources of tropical forests. CIFOR is currently studying these relationships.

Human activities in forests modify the size and age structure of the tree species, and potentially alter genetic structure and levels of genetic diversity. Disturbance usually involves harvesting a variety of products from the forest: wood, fruit and other foods, medicinal plants, construction materials, and many more. "Disturbance" is used here to mean the impact of human activities on forests, as distinct from "natural perturbations" caused by wind, water or natural fire. Commercial harvesting of timber through logging is a major form of disturbance in many areas, and CIFOR is investigating the impacts of commercial logging in Asia, Africa and Central America. However, the results presented here come from studies focusing on other forms of disturbance, such as extraction of NTFPs, fire, or the cutting of wood for local consumption.

As use increases, genetic diversity of many species will be affected, with those species unable to tolerate disturbance becoming locally extinct. In order to encourage conservation of biodiversity, it may be necessary to offer financial compensation to those who adapt their harvesting practices to favour biodiversity.

Harvesting products from the forest may affect genetic diversity directly or indirectly. Gathering NTFPs can affect selection, migration and mating systems. For example, harvesting of reproductive structures (fruits, nuts) can affect migration by reducing seed dispersal, modifying the mating system, and may also impose a selection pressure.

The impacts of harvesting will vary according to the "life history strategies" of the species and the type of product harvested, as is demonstrated by research in Thailand and India. CIFOR research partnerships are sampling 6-8 species under different levels of disturbance on two sites.

In Thailand the major study site is a Wildlife Sanctuary and World Heritage Site at Huay Kha Khaeng. The research includes two species of dipterocarp, Shorea siamensis and Dipterocarpus obtusifolius, that are contrasting, both in terms of ecology and economic uses. Shorea siamensis is cut by local people for construction timber. It is predominantly pollinated by a species of weak-flying bee. As disturbance increases (see figure opposite) the density of mature trees falls by a factor of 10. At the same time, the density of flowers per tree increases dramatically. There appears to be a threshold point where the distance between adjacent flowering trees exceeds the distance normally covered by the pollinating bee. Consequently, reproductive success and outcrossing rates decline rapidly, with a related decline in genetic diversity, both among the embryos and in mature individuals.

In contrast, outcrossing rates for Dipterocarpus obtusifolius do not decline at disturbed sites. The effective control of fires within the wildlife sanctuary may increase the loss of flowers and fruit to insects and other pests, thus reducing outcrossing. It would seem that Dipterocarpus obtusifolius is a truly fire-adapted species. The consequences of controlling fires and illegal cutting within the wildlife sanctuary therefore seem to favour Shorea siamensis at the expense of Dipterocarpus obtusifolius.

The most disruptive influence of harvesting is likely to be removal of reproductive structures (flowers, fruit and seeds). In southern India, fruits of Phyllanthus emblica are harvested for food and as the base for many traditional medicines.

The villagers in Thailand are best described as forest-margin dwellers. However in India, research has focused on the activities of true forest dwellers ­ an indigenous tribal group, the Soligas. Research has shown that the Soligas rely on NTFPs for more than 60 per cent of their income. Previously, when living a semi-nomadic existence, their use of NTFPs was probably sustainable. Since being obliged to settle in permanent villages, the collection activities have been concentrated in fewer areas within which the genetic diversity of valuable species has been significantly affected. Reproduction of some of these species has been prevented due to over-harvesting of fruit and seeds. Although other factors may have contributed to these changes, it does seem that the policy-driven change in social activities has adversely affected the resource, at least near villages. As older trees die, the economic viability of the Soligas will also be severely affected as they are so dependent on forest resources.

Samples collected from two areas were subjected to three different intensities of collection and separated by only a few kilometres. They show distinct differences in characteristics such as germination percentage, seedling vigour and seed predation. The samples collected from the most disturbed site had the lowest germination percentage and vigour and the highest rates of seed predation. A Principal Component Analysis of seedling phenotypes shows clear differences between the seedlings at the three levels of disturbance. This may simply be due to physical isolation, but the way Phyllanthus emblica reproduces (wind pollination and animal seed dispersal) means this is unlikely.

Many integrated conservation projects in areas of high biodiversity have failed because the link between economic activities and resource conservation has not been understood. The impacts of disturbance are not easily predicted, and will depend on a variety of factors. These include intensity, harvesting cycles and the life history characteristics of the species affected. Harvesting of most products, if carefully regulated, can be undertaken without significant long-term impacts on genetic diversity. Harvesting of reproductive structures for food or medicine is likely to have the most significant impact on genetic diversity, due to the direct consequences of removal of fruits or seeds, and the difficulties in regulating the harvest. Research on these genetic processes can help achieve a balance between the needs of the forest-dependent people and the genetic integrity of the resource.

Timothy Boyle